Radiological weapons (e.g., dirty bombs) are a threat to national security, and a means of detecting them before they are placed at their target is desirable. Ideally, the weapons or their radiological precursors should be detected as they are transported on highways, while still far from their target. However, because the radiological sources used in these weapons are relatively low intensity, and current radiation sensors have relatively low sensitivity, identifying these radiological sources in moving traffic is difficult, and subject to false alarms due to naturally occurring background radiation.
In general, cars move past fixed sensors too quickly to enable the sensors to integrate over enough time to create a clear signal. The current approach to detection of sources in traffic is to place sensors at points where traffic is expected to be moving slowly (e.g., at traffic signals).